```ts
const MODULUS = 1000000007;

/**
 * The nth Fibonacci number, modulo 1000000007.
 *
 * @param n Non-negative integer representing the position in the Fibonacci sequence.
 *
 * @throws Error if n is not a non-negative integer.
 */
export function fibMod(n: number): number {
  if (!Number.isInteger(n) || n < 0) {
    throw new Error('Invalid input. n must be a non-negative integer.');
  }

  let a = 0;
  let b = 1;

  for (let i = 2; i <= n; i++) {
    const temp = (a + b) % MODULUS;
    a = b;
    b = temp;
  }

  return b;
}
```